Unbalanced-to-balanced impedance



Nov. 14, 1950 c. DRISCOLL 2,530,048

UNBALANCED-TO-BALANCED IMPEDANCE MATCHING DEVICE Filed Dec. 10, 1945 INVENTOR.

CLARE DRISCOLL BY wwfi ATTORNEY Patented Nov. 14, 1950 UNBALAN CED -TO -BALANCED IMPEDANCE MATCHING DEVICE Clare Driscoll, Brookline, Mass., assignor to the United States of America as represented by the Secretary of War Application December 10, 1945, Serial No. 634,103

. 4 Claims. 1

The present invention relates to a device for transforming an unbalanced transmission feed line to a balanced line through which a balanced antenna or other balanced load can be properly energized. The antenna for which this device is principally used may be a balanced dipole antenna consisting of two identical arms or'se'ctions which aredesired to be energized with equal and opposite voltages.

The usual type of transmission feed line is a concentric or coaxial transmission line characteristically unbalanced. With such a feed line, there is an impedance mismatch between the antenna and the transmission line which results in reflections taking place at the junction between the antenna radiating elements'and the transmission line feeding the same, whereby part of the energy is reflected back into the transmission line and standing waves are likely to beset up. The ratio of the amplitudes from the maximum to the minimum of the standing wavesset upat the junction increases with the amount of mismatch at the junction. Such impedance mismatch results in reducing the energy transfer characteristics of the antenna and increases the frequency sensitivity.

It is one of the objects of the present invention to overcome these disadvantages and to provide a device which will transform an unbalanced transmission line to a balanced line through which a balanced antenna or other balanced load can be properly energized.

It is another object of the invention to provide a device for reducing the standing wave amplitudes in transmission lines for feeding energy to a balanced load.

It is another object of the invention to provide a device designed so that the index of impedance mismatch between a transmission line and a load, such as an antenna, is maintained at a low value over a wide range of frequencies.

It is still another object of the invention to provide a device having a length such that its characteristic impedance will match the characteristic impedance of the load fed by the transmission line and particularly in which such length is either a substantial fraction of or slightly more than a quarter wavelength at the center frequency of the desired range of operation.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing in which:

The single figure is a front elevation view of the transformer device according to the present invention.

In the drawing, the novel transforming device according to the present invention is generally designated III. The transformer It! comprises a coaxial or concentric conductor transmission line I I which may be connected to, or be an extension of, or in alignment with a concentric transmission line I2 which is connected to a source of electromagnetic energy. An enclosed transmission line I3, preferably a hollow pipe of circular cross-section, is disposed parallel to line II and is of substantially equal length therewith A shorting member or bar I4 connects one end of line I3 with the outer conductor of line H. 'At the opposite end of line I 3, a strap or other electrically conductive member I5 connects the line I3 to the inner conductor I6 of line II. A load may be connected to' the free ends of lines I I and I3. By way of example and for simplification of description, the load is shown as a balanced dipole antenna comprising substantially identical dipole sections I1 and I8. Dipole section I1 is electrically connected to the outer conductor of line H and dipole section I8 is electrically connected to line I3. A sleeve or shield I9 is pro vided to surround lines II and I3, for substantially the length thereof to prevent radiation and transmission losses.

The length a: of the transformer device I0, from the shorting bar I4 to the dipole section II, is either a substantial fraction of or slightly more than a quarter wavelength at the center frequency of the desired frequency range of opera tion. The length :1; of each dipole section I! and I8, or each half of a balanced load to which the transformer device It is connected, should'be greater than a quarter wavelength at this'middle frequency of operation when the length a: is less than a quarter wavelength, or the dimension y should be less than a quarter wavelength when the length a: is'greater than a quarter wave-' length.

It will be noted that the transformer device In has a length other than a quarter wavelength; if the length at were a quarter wavelength at the center frequency, the impedance would be infinite at the center-frequency. However, as hereinbefore described, the transformer I-Il shunts, by means of member I 4, a reactive 1m: pedance across the lines I I and I3 when its length :0 varies from a quarter wavelength of the frequency to be transmitted. The band of frequencies which could successfully be radiated therefore would be narrow if the impedances of the transformer device 10 and of the antenna or load were matched at a quarter wavelength and no compensation were made for the reactive impedance at other frequencies. For example, the length of transformer device l may be approximately .2. wavelengths at the center frequency and each of dipole sections l1 and I8 of the antenna may be approximately .3 wavelengths in length. This mismatch causes some reflection of energy at the center frequency. However, at frequencies above and below the center frequency, the inductive reactance, and capacitive reactance of the transformer Ill and the antenna or load partially cancel and there is less reflection. The range of frequencies, therefore, over which this device In and its load or antenna can satisfactorily operate is increased over that of a conventional unit. Substantially the same results would be eiTected if the dimension :1: were approximately 0.3 wavelengths at the center frequency and the dimension y were 0.2 wavelengths.

Thus, it will be seen that for a balanced antenna (or other load) comprising two identical sections or arms i1 and I8 which must be energized with equal and opposite voltages, the'device l0 according to the present invention may be used to transform the feeding concentric line into a balanced line so that the antenna may be properly energized, that is, so that equal and opposite voltages may be applied to the load or the antenna sections !1 and I8, while the standing wave ratio may be minimized. From plotted graphs of the standing wave ratio versus frequency, it has been shown that with the transformer device according to the present invention, the standing wave ratio is 'less than 2:1 over a wider band of frequencies than that obtained when both the load and transformer device are exactly a quarter wavelength at the center frequency of the band utilized.

While there has been here described what is at present considered the preferred embodiment of the invention, it will be ObViOlls to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. A system for minimizing standing waves in a broad band radio frequency transmission system wherein a balanced load is matched to an unbalanced line including in combination a dipole antenna each of whose elements differs inlength by substantially .05 wavelengths from one quarter Wavelength at the center frequency of the desired frequency band of operation and a transformer device comprising a section of coaxial transmission line having a predetermined length and having an inner conductor and a conadditional conductor substantially equal in length to said coaxial line length and disposed in spaced parallel relation to said coaxial line to form a balanced two conductor transmis ion line with said outer conductor, the length of said additional conductor and said coaxial line differing from said quarter wavelength in the opposite direction from and by the same amount as the length of said dipole elements and a shorting member connecting the first end of said additional conductor to the outside of said outer conductor at the first end of said coaxial line, the second end centric outer conductor spaced therefrom. an

of said additional conductor being connected to said inner conductor at the second end of said coaxial line, one of said dipole elements being connected to the second end of said additional conductor and the other of said dipole elements being connected to said outer conductor at the second end of said coaxial line whereby the reactance of said dipole and said transformer device substantially cancel above and below the center frequency of the operating range.

2. The system set forth in claim 1, wherein the lengths of said dipole elements are each 0.3 wavelength at said center frequency and said balanced line length is 0.2 wavelength at said center frequency.

3. The system set forth in claim 1, wherein the lengths of said dipole elements are each 0.2 wavelength at said center frequency and said balanced line length is 0.3 wavelength at said center frequency.

4. A system for minimizing standing waves in a broad band radio frequency transmission system wherein a balanced load is matched to an unbalanced line including in combination a dipole antenna each of whose elements differs in length by substantially .05 wavelengths from one quarter wavelength at the center frequenc of the desired frequency band of operation and a transformer device comprising a section of coaxial transmission line having a predetermined length and having an inner conductor and a concentric outer conductor spaced therefrom, an additional conductor substantially'equal in length to said coaxial line length and disposed in spaced parallel relation to said coaxial line to form a balanced two conductor transmission line with said outer conductor, the length of said additional conductor and said coaxial line differing from said one quarter wavelength in the opposite direction fromfand by the same amount as the length of said dipole antenna elements, a shield surrounding saidbalanced line for substantially its entire length and a shorting member Connecting the first end of said additional conductor to the outside of said outer conductor at the first end of said coaxial line, the second end of said additional conductor being connected to said inner conductor at the second end of said coaxial line, one of said dipole elements being connected to the second end of said additional conductor and the other of said dipole elements being connected to said outer conductor at the second end of said coaxial line whereby the reactance of said dipole and said transformer device substantially cancel above and below the center frequency of the operating range.

CLARE DRISCOLL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

